This invention relates to a belt drive system for advancing print media through a printer.
An ink-jet printer includes at least one print cartridge that contains ink within a reservoir. The reservoir is connected to a print head that is mounted to the body of the cartridge. The print head is controlled for ejecting minute droplets of ink from the print head to a sheet of print medium, such as paper, that is advanced through the printer.
Many ink-jet printers include a carriage for holding the print cartridge. The carriage is scanned across the width of the paper, and the ejection of the droplets onto the paper is controlled to form a swath of an image with each scan. Between carriage scans, the paper is advanced so that the next swath of the image may be printed.
Oftentimes, especially for color images, the carriage is scanned more than once across the same swath. With each such scan, a different combination of colors or droplet patterns may be printed until the complete swath of the image is formed.
In some printers, an array of print heads may be provided to extend across the entire width of the paper that moves through the printer. As is the case with scanning type printers, the relative position of the print heads and paper must be precisely maintained to effect high-resolution, high-quality printing. This precision is especially important in the region known as the xe2x80x9cprint zonexe2x80x9d of the printer, which is the space where the ink travels from the print head to the paper. Changes in the relative position of the print head and paper will cause the expelled ink droplets to land imprecisely on the paper and thus degrade the quality of the printed image.
One method of securing a sheet of paper for movement through a printer is to direct it against an outside surface of a moving carrier such as a hollow cylindrical drum. The curved drum wall has holes through it. The interior of the drum is partially evacuated and the resultant suction is communicated through the holes in the drum to the underside of the paper to thus hold the paper against the drum. The drum is rotated to move the sheet through the print zone to receive the ink. This suction technique for securing the paper to a carrier can be designated generally as xe2x80x9cvacuum hold.xe2x80x9d
Using a drum-type carrier with vacuum hold has at least a couple of disadvantages. For one, relatively stiff paper will resist bending from its normally flat shape to conform to the curved circumference of the drum. Thus, to adequately secure the stiff paper to a drum, the vacuum pressure must be increased, which increases the power required for vacuum hold of stiff paper.
Further, the printer volume required to house a drum can be quite large, especially for printers that are intended to handle large-format print media. It is always desirable to make a printer as compact as possible, without sacrificing performance of the printer.
One way of overcoming the disadvantages associated with drum printers is to employ a perforated belt, one side of which carries the paper. Vacuum pressure is applied to the other side of the belt and thus through the perforations to secure the paper to the belt. The belt, with secured paper, is moved relative to the print head and through the print zone where ink is printed to the paper.
The belt may be configured as an endless loop and secured between a pair of rollers that drive it. The upper surface of the belt between the two rollers can be used for transporting the paper, which can be directed to and removed from the upper surface of the belt in the vicinity of the rollers. Thus, the diameter of the rollers can be minimized so that the belt provides a compact, vacuum-hold-type of print media carrier.
Since a belt-type media carrier is inherently flexible, mechanisms must be employed to ensure that the belt is precisely supported relative to the print head(s). Using a stationary support mechanism for this purpose, however, introduces problems with friction. That is, the belt must be adequately supported by the stationary mechanism in a manner that does not generate significant frictional forces. Too much friction can damage the belt, cause it to slip, or wear.
In addition to addressing these friction problems, a belt-type carrier and associated support mechanism must be constructed so that the vacuum pressure applied through the belt to the paper is evenly distributed. In this regard, the suction or vacuum pressure (here the term xe2x80x9cvacuumxe2x80x9d is used in the sense of a pressure less than ambient), must be applied in a manner that ensures that the sheet of paper remains in contact with the belt. If, for example, the edges of the sheet lift from the belt as a result of too little vacuum pressure, there is a likelihood that the paper will contact the print head, which is quite undesirable.
If the vacuum pressure level is unevenly distributed, or if the paper is not well supported, the surface of the sheet may become deformed in the places where the relatively high vacuum pressure or inadequate paper support occurs. Such deformations will change the relative position of the paper and print head, and thus lead to the above noted degradation in print quality.
The present invention is generally directed to a belt-type media support for print media that generally overcomes the disadvantages that arise with drum-type carriers and addresses the other problems just discussed. In short, the invention provides a flexible belt that is supported in a manner such that friction is minimized and so that the paper supported on the belt is advanced precisely through the print zone of the printer. A uniform distribution of vacuum pressure is applied to the paper carried by the belt.
One preferred embodiment of the invention features a rigid, flat platen for supporting a substantially porous belt that carries the paper. The platen has a number of rollers mounted to its surface to minimize the friction developed between the platen surface and the moving belt. The platen surface is configured to provide a uniform distribution of vacuum pressure to the porous belt, which pressure is channeled to the paper carried by the belt.
In another embodiment, specially configured sliders are carried on the support surface of the platen. Like the rollers, the sliders reduce the area of contact with the belt to thus reduce friction. Also, the sliders are shaped to limit the amount of area of the belt to which vacuum pressure is applied, thereby to minimize frictional forces while supporting the belt in a position spaced from the platen support surface. The sliders combine with a belt for communicating vacuum pressure from the support surface of the platen through aligned holes in the otherwise non-porous belt. The vacuum pressure is channeled to the paper that is carried on the belt.
In yet another embodiment, the friction between the belt and the substantially flat platen surface is minimized by a coating of low friction material applied to at least the surface of the belt that contacts the platen surface. This platen surface preferably includes protrusions that are mounted to the platen surface and that define the channels for distributing vacuum pressure to the belt.
Other advantages and features of the present invention will become clear upon review of the following portions of this specification and the drawings.